Gas tube with reduced noise



A. L. TIRICO 2,687,485

GAS TUBE WITH REDUCED NOISE Filed April 2, 1951 2 Sheets-Sheet 1 A'ITORNEY Aug. 24, 1954 T|R|Q 2,687,485

GAS TUBE WITH REDUCED NOISE Filed April 2, 1951 v 2 Sheets-Sheet 2 INVEN'EOR Arthur L. 7'11'100 ORNEY Patented Aug. 24 1954 UNITED STATES PATENT OFFICE 2,687,485 GAS TUBE WITH REDUCED NOISE Arthur L. Tirico, Belleville, N. J., assignor to Radio Corporation of America,

of Delaware Application April 2, 1951,

12 Claims.

This invention relates to the reduction of noise in gas tubes and preferably in gas tubes which are suitable for use as amplifiers because they have continuous grid control. More particularly, it relates to the reduction of gas tubes of a particular kind having very high values of transconductance and of anode current and extremely low values of output impedance filed on September 20, 1950, September 20, 1950, and March 28, 1951, and all of which were assigned to the same assignee as the present application, and to novel circuits for reducing noise in gas tubes.

Tubes of the particular kind alluded to above, which are referred to as plasmatrons, have offered very great advantages over gas tubes which operating principles of be reviewed. In this kind of tube there are separate discharge paths for the load current and the ionizing current. The energizing potential required for drawing the former, the load current, from a main cathode to a main anode, is

It ionizes the tubes gaseous filling converting it into a conductive plasma consisting of positive ions and detached, free negative The plasma (1) surrounds the main is very large, e. g. ampere, despite the very low main anode potential, e. g., 5 volts. As a result a, load-current multiplication-effect results from the use of the auxiliary discharge.

a. corporation Serial N 0. 218,744

this end the auxiliary discharge is energized with a steady direct potential and no measures are taken to modulate it. Moreover, the input signal is applied to a load current control grid which is located in the path of the low velocity load current where it Will not influence the plasma density. In these tubes it is the fact that a consheath varies in thickness; and this changes the cross-sectional areas and hence the conductivity of columns of conductive plasma which extend through the grid openings between the main cathode and the main anode. The attainment and retention of this type of load current control in a gas tube marked a great advance. In most other kinds of gas tubes, e. g., thyratrons, condensity are intermixed with the same kinds of spurious fluctuations and random variations.

Any of these tubes in either of these classes can be considered as comprising a group of electrodes for carrying the load current and a gas diode for generating the plasma. The source of electrons for the diode is the auxiliary cathode and its anode, or collector of electrons, is either an electrode especially provided for this purpose or one of the electrodes of the load current circuit, e. g., the main cathode, or a group of those electrodes serving as a composite anode. In any case the operation of the auxiliary discharge is substantially the same as that of an ordinary gas diode. At the instant when a gas diode fires its electron space discharge becomes abruptly augmented by a very large conduction current. There is a short interval in which a regenerative increase in current takes place and the diode tends to run away. During this interval the greatly increasing electron flow greatly increases the generation of plasma which in turn further increases the electron flow, and so forth. Excluding the academic possibility that the external energizing circuit has zero series impedance, the large current inevitably will cause enough of an IR drop in it to tend to extinguish the discharge. However, the plasma decays rather slowly so that large current and therefore the drop in the applied potential persists for a very sensible period of time. When this period has passed the applied potential will be restored and the next regenerative interval, 1. e., a fresh cycle, will commence. Thus a gas diode connected to a source of energizing potential is inherently a relaxation oscillator. As a result the discharge current, and with it the plasma density, is inherently subject to gross periodic fluctuations. These are sometimes referred to as plasma oscillations. In a plasmatron the plasma density fluctuations effect very substantial fluctuations in the output current.

In addition there are other variations which are more random in their nature. They are probably attributable to the random distribution of the velocities and directions-of-emission of the thermal electrons provided by the auxiliary cathode as those random phenomena are translated into the generation of ions and probably the average recurrence rate of the random gas fluctuations is probably related to the average recurrence rate of the thermal noise impulses by some factor which depends on the average number of ionizing collisions and therefore on the magnitude of the arc-drop. Thus gas discharge random noise is often coarser than the thermal noise encountered in hard tubes.

It is therefore an object of the present invention to reduce the generation of noise in gas diodes generally as well as in the types of tubes described above each of which inherently includes a gas diode as a source of plasma.

It is a further object of this invention to reduce both the fluctuation noise and the random gas discharge noise of a gas tube.

In general the objects of the present invention are attained by providing, either within a gas diode or plasmatron or external thereto in the circuit for energizing a gaseous discharge therewithin, a constant current means for so limiting the discharge current that the relaxation-oscillation phenomenon is either markedly reduced or eliminated and that the sustaining potential for the gaseous discharge device will have a value which is adverse to the generation of coarse random gas noise.

In the drawing: Figure l is a longitudinal sectional view of an illustrative embodiment of a tube according to the present invention, the section is taken alon lines I--! of Figure 2 and in a plane which is parallel to the axes of the cylindrical cathodes;

Figure 2 represents another longitudinal sectional view of this embodiment, this section being taken along the line 2-2 of Figure l in a plane perpendicular to that of the section of Figure 1;

Figure 3 is a longitudinal sectional View of a modification of my invention, the section being taken like that of Figure 2;

Figure 4 is a longitudinal sectional view of another modiflcation, the section being taken as in Figures 2 and 3;

Figures 5, 6 and '7 are schematic circuit diagrams of amplifier circuits respectively using the tubes of Figures 1 and 2, 3, and 4, and

Figure 8 is a schematic circuit diagram of a rectifier circuit using a gas diode improved according to the present invention.

The as tube 10 shown in Figures 1 and 2 comprises a gas tight envelope 1 i which in this illustrative embodiment is of substantially rectangular shape. The load current in this tube originates from a main indirectly heated cathode I2 and is received at a main anode l3. An auxiliary indirectly heated cathode It serves as a source of electrons for the ionizing discharge. A load current control g-rid I5 is mounted intermediate the main cathode and anode. A fine mesh screen it is mounted between the auxiliary cathode l4 and the group of load current electrodes l2, l3 and I5. Preferably the screen l6 should be of very fine mesh, for example, with openings which are of the same order as the mean free path of a positive ion and preferably the screen should completely separate the space within the envelope which contains the auxiliary cathode 14 from that containing all of the load current electrodes. In the particular embodiment shown herein the screen 16 is mounted within a rectangular frame ll which is closely fitted to the inside surface of the envelope I I. However this exact arrangement is not necessary. Instead the frame ll could be smaller than the cross-sectional area of the envelope II and in the form of a cage entirely surrounding the auxiliary cathode; or it could be in the form of a solid, i. e., non-foraminous, partition with the fine mesh screen covering only a relatively small central opening. The main and auxiliary cathodes l2 and M are supported respectively on rods I8 and H! which extend through the top of the envelope l i to serve as terminal pins over which the sleeves of the cathodes can be polarized. Respectively associated with these cathodes, there are two additional terminal pins 20 and 2| which serve for completing the connection of the heaters of these cathodes to sources of current. As shown in the drawing each of the cathodes l2, M has an internal heater, 22, 23 respectively, which is connected on one of its ends to the cathode sleeve and on its other to the terminal pin associated with that cathode. A terminal pin 24 extends through the bottom of the envelope H to the frame H to provide a means for connecting the screen it to an external circuit. A similar pin 25 extends through the bottom of the envelope H to the control grid 15. Because of the large size of the control grid l5 shown herein by way of example, a pair of rods 26 are employed to assist the terminal pins 25 in supporting the grid. Likewise the horseshoe shaped main anode I3 is carried on a terminal pin 21 and a pair of support rods 28.

Tube It may be processed in any number of tube due to its tendency to run away. Since W ys well known in the art .to provide a gaseous the principal feature of the present invention in- Any suitable gas .or mixture of gases may be of a constant current means either inside of the utilized. The gas pressure for any particular em- 5 tube or in its aux l ry di h r zin bodiment will be accordance with its specific cu t, the resistor 33 y be dispensed Withelectrode geometry and spacings and must be As pointed out above, in previous plasmatrons such to favor the formation of a self sustainin the current passing between the auxiliary cathionizing discharge. A number of plasmatron ode l4 and the composite anode l2, l3, l5 has tubes have been found to operate satisfactorily been subject to current fluctu ions Figure with a filllng of h lium of a pressure of approxis ws two Ways of supp s e th s flu tuati ns mately 750 microns. However, as is well known e first s o adlllet the temperature of t other gases and other pressures may b used, auxiliaiy cathode it so that for the particular 6 g, pressures which lie within the range of beopelatlhg hotentlals pp t e t e In t tween approximately 100 microns and several emission is temperature limited. To this end millimeters of mercury. the heater circuit connecting the source 33 to Figure 5 shows a, circuit i hi h th embodithe cathode id includes a potentiometer 38. m nt of Fi ures 1 and 2 may be employed a When the use of temperature limited emission is four direct potential sources are shown, a low- 0 charge, this will in itself sufiice and therefore it potential load-.current-energizing source 30; a, Will be possible to dispense with the use of the high potential auxiliary-dischar e-energizin screen or to pelerize t at a i n y hi h source 3!; .a heater current source as for the potential to draw away from e auxiliary cathauxiliary cathode l4; a d a biasingource 32 for ode Hi all of that limited emission which it does the main control grid L5. The source 30 is used provide. However, a second and preferred way of series-parallel onn t 1 volts dry cells, a the use of temperature limited emission. In

o a b t r or any similar fairIy-high-curdoing this the auxiliary cathode may be heated substantial sjgn al-bearingcurrent e" acurrent 5 i6 adjusted to a positive pot ntial of some order of on t al amperes conversely the vides so low a field gradient between itself and source 3| h m b capable of providing a re1a the cathode that space charge limitation will set tively much higher potential but need not be in. Under such conditions a limited current of capable of providing a particularly lar e contin 40 electrons will be attracted toward the screen I6 0115 average current, This is because it serves to and a portion of this, also limited and constant, energize the efiicient, low-current ionizing-dis- W111 s r gh its open ngs toward the comcharge Theskource 33 is merely a heater current posite anode l2, l3, #5. In effect the cathode l4 supply, F u t Source 32 is only called upon and the screen i 6 when operatin together in this t provide l voltage and that with little way constitutes the equivalent of a virtual temrent since t merely provides a small negative perature limited cathode. When the potential bias between the control grid I5 and the auxiliary provided by the source 35 is sumcient to the main anode is nearly so since the source 30 i i w and to maintain a going are, the .estabhshe only small voltage difierence auxiliary discharge circuit will be functioning in with the sour e 30, t main cathode l2 and the large number of ionizing collisions. This certmnsmrmer 34 coupling the varying component seems in addition to cause the random noise imreceiving electrons from the auxiliary cathode I 4 6 by intelligence signals: the audio bandlarity that the auxiliary cathode is sufliciently I191; be disturb by the. Sudden generation of towards the auxiliary cathode can occur, the rate of this diffusion can be made very low with respect to the rate at which they will recombine with free electrons (in the region to the left of the screen) if the openings are very small. An incidental advantage resulting from the use of the screen I6 is that back bombardment of the auxiliary cathode by positive ions is almost entirely eliminated.

In Figure an arrangement is shown at 39 whereby it is possible to adjust the potential of the screen [6. In addition, a conventional type of signal input circuit is shown at 40 for superimposing signal modulations on the bias of the load current control grid l5. If desired the same signal may be applied to the screen It instead of the grid l5 by substituting the circuit represented in dotted lines at 4! for the circuit 40. Or, where .a dual input device is needed, both circuits may be used.

Many of the elements comprising the embodiment of Figure 3 correspond to above-described elements comprising that of Figure 1 and they are designated by the same reference numerals. However, in this embodiment an auxiliary-discharge control grid 29 is placed in cooperative relationship between the auxiliary cathode l4 and the screen l6 and the load current control grid I5 is eliminated. In such an embodiment the electrodes providing the auxiliary discharge, i. e., the cathode M, the grid 29, and the screen [6 and those comprising the composite auxiliary anode together constitute an arrangement very much like a tetrode. In this type of tube the load current is controlled indirectly by modulating the plasma density.

Figure 6 shows a circuit for utilizing the tube of Figure 3. This circuit contains many elements like elements of the Figure 5 circuit and these bear the same reference numerals. However, in this circuit it is the control grid 29 which is provided with a signal input circuit (43) and a biasing arrangement (42) whereas in Fig ure 5 the corresponding circuit portions 32 and 4B are provided for the grid i5. If desired an embodiment may be made which includes both a load current control grid l5 and an auxiliarydischarge control grid 29 to afford a dual input device. In such a case an appropriate circuit would include the circuit portions 32 and 40 as well as those shown at 43 and 42.

The embodiment of Figure 4 comprises a plasmatron of the kind described in copending appli cation, Ser. No. 217,912, now U. S. Patent No. 2,611,884. In this kind of plasmatron the ionizing discharge is formed into a directed stream having a constricted portion of smaller cross-sectional area than the rest of the stream and a means is provided for constriction-modulating the ionizing stream to vary the plasma density and thereby the load current. In this Figure 4 embodiment a number of the tube elements are like the elements used in the embodiment of Figures 1 and 2 and that of Figure 3 and therefore they bear the same reference numerals. In Figure 4, however, the load current control means comprises a beam-forming slotted shield 50 and a constriction modulating electrode 5!. The electrode 5! has a slot 52 which is aligned with the slot (53) of the shield 50. For a complete explanation of the constriction-modulating principle of operation, reference is made to the above-mentioned. copending application. In Figure 4 the required fine mesh screen has the form of a cage 54 which surrounds the auxiliary cathode M. It is operated in the manner above described for the screens l6 to provide a constant auxiliary current.

Figure '7 shows a circuit as for utilizing the tube of Figure 4. In this circuit it is the constriction modulating electrode 5| to which the input signal is applied and accordingly there is a circuit portion consisting of biasing means 55 and an input means 56 to take the place of corresponding circuit portions shown in the circuits of Figures 5 and 6. In the operation of the circuit of Figure 7 the electrode 5|, being negatively biased, collects and becomes surrounded by a sheath of positive ions. As a result variations in the direct potential bias of this electrode are effective to vary the thickness of this sheath and thus to modulate the constriction in the ionizing stream. These constriction-modulations will vary the arc drop and hence the plasma density, and as a result they will modulate the load current. However, the useful signal variations in the plasma density will not be accompanied by the usual plasma fluctuations. In each of the circuits in Figures 5, 6, and '7 there is shown a block L which is intended to represent an external current limiting means which may be used in addition to the current limiting means already described or in place thereof. An example of a suitable external current limiting means would be a pentode tube with its plate and cathode connected in series with the source 3| and the auxiliary cathode i4.

Each of the tubes shown herein in effect comprises as a subcombination a gas diode which serves to generate plasma and is modified to reduce its generation of spurious noise. Such a gas diode has great possibilities for usefulness without being used in a plasmatron, for example, it may be used as a rectifier. The circuit of Figure 8- represents a rectifier circuit which employs such a diode and may be suitable, for example, for changing a volt alternating current voltage into a direct potential of perhaps 300-400 volts. The circuit comprises a gas diode 60 having an indirectly heated cathode 5i, anode 62, and a fine mesh screen 63 positioned in the tube between these elements to function like the screens l6 and 54 described above. A transformer 64, which serves to step up the peak-to-peak value of the alternating voltage to a value which is large enough for the desired D. C. voltage, has its secondary 65 connected between the cathode 6! and the anode 62. The screen E53 is connected to a tap of the secondary 55 so that during the half cycle when the rectifier 60 conducts the instantaneous potential of the screen will be within a small range of positive values, with respect to the cathode 6!, which are suitable for keeping the current space charge limited and free of spurious fluctuations. of course, if desired, a D. C. bias can be provided between the screen 53 and the cathode 6i. Each pulse of current drawn through the gas tube 50 will produce a voltage drop across a resistor 66 and a filter 61 will act in a known manner to derive the D. C. component of this voltage and to supply a stored source of direct potential in the reactive elements of the filter.

The screen It does not necessarily have to completely isolate from the rest of the inside of envelope -i l, the portion thereof which contains the auxiliary cathode. If there were an open space all around the perimeter of the screen It between it and the adjacent inside surfaces of the envelope it would still easily perform its desired function of electrically shielding the auxiliary cathode and it might also perform its additional function of keeping positive ions from 1. A gas amplifier tube comprising: a sealed envelope containing a gaseous filling; means for the tube @0111- includes a fine mesh screen between said auxiliary cathode and said slot.

3. A gas tube as in claim striction-modulating means substantial registry with that 4. A gas amplifier tube comprising: a sealed 5. A gas amplifier tube comprising: a sealed envelope containing a gaseous filling; means for cooperative spaced relationship; means including an auxiliary cathode for producing a currentdischarge to grid adjacent said auxiliary cathode.

6. A tube as in claim and further comprising a control grid between said main cathode and said main anode.

7. A gas amplifier tube comprising: a sealed envelope containing a gaseous filling; means for carrying a load current through the an auxiliary stabilized ionizing discharge to provide a conductive plasma between said main cathode and said anode; said second-mentioned means including a fine mesh screen between said auxiliary auxiliary cathode and between said auxiliary cathode and said screen.

8. A gas amplifier tube comprising: a sealed envelope containing a gaseous filling; means for carrying a load current through the thereto, for example, coincident therewith or crosswise or adjacent thereto, that said ionizing discharge bathes the main cathode and extends over all of said current path; said last-mentioned ative spaced relationship; auxiliary cathode and a fine mesh screen adja- References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,999,736 Morrison Apr. 30, 1935 2,100,195 Lowry Nov. 23, 1937 2,146,652 Reed Feb. 7, 1939 2,158,564 Meier May 16, 1939 2,218,551 Nelson Sept. 3, 1940 2,383,492 Klemperer Aug. 28, 1945 2,428,661 Fitzmorris Oct. 7, 1947 2,436,835 Stutsman Mar. 2, 1948 2,578,571 Meier Dec. 11, 1951 

